Among rare earth magnet material alloys which have been used in recent years are R-T-B-based alloys, which exhibit excellent magnetic properties. In the term “R-T-B-based alloys” as used herein, “R” refers to rare earth metals, “T” refers to transition metals with Fe being an essential element, and “B” refers to boron. An alloy, made of such an R-T-B-based alloy, which serves as a material for rare earth magnets can be produced from a ribbon cast by a strip casting method.
FIG. 1 is a schematic diagram of a casting apparatus that is used for casting of ribbons using a strip casting method. The casting apparatus shown in FIG. 1 is provided with a chamber 5, a crucible 1, a tundish 2, and a chill roll 3. The inside of the chamber 5 is maintained to be in a reduced pressure condition or an inert gas atmosphere, whereby oxidation of the molten alloy and the cast ribbon is prevented.
When a ribbon of an R-T-B-based alloy is cast by a strip casting method using such a casting apparatus, the following procedure, for example, may be employed.
(A) Raw materials are loaded into the crucible 1, and the raw materials are heated using an induction heating apparatus (not shown). Thus, the raw materials are melted to form a molten alloy.
(B) The molten alloy is supplied to the outer peripheral surface of the chill roll 3 via the tundish 2. The chill roll 3 is configured to have a coolant circulating therein, and therefore the molten alloy is rapidly cooled on the outer peripheral surface of the chill roll 3 to be solidified.
(C) In this manner, a thin ribbon 4 having a thickness of 0.1 to 1.0 mm is cast. The chill roll 3 rotates in the direction shown by the hatched arrow in FIG. 1 and accordingly the ribbon 4 separates from the chill roll 3.
The thin ribbon cast by a strip casting method is crushed into alloy flakes and then they are cooled under predetermined conditions. The crushing of the ribbon and the cooling of the alloy flakes are typically carried out under reduced pressure or in an inert gas atmosphere in order to prevent oxidation of the alloy flakes.
The resultant R-T-B-based magnet material alloy (hereinafter also simply referred to as “magnet material alloy”) has a crystal structure in which a crystalline phase (principal phase) of R2T14B phase and R-rich phases having concentrated rare earth metals coexist. The principal phase is a ferromagnetic phase that contributes to magnetization, and the R-rich phases are non-magnetic phases that do not contribute to magnetization.
An R-T-B-based magnet material alloy is also referred to as an Nd—Fe—B-based magnet material alloy because R is mainly composed of Nd and T is mainly composed of Fe. Magnet material alloys are widely used as materials for R-T-B-based sintered magnets and R-T-B-based bonded magnets, and of these, R-T-B-based sintered magnets are also referred to as neodymium sintered magnets.
R-T-B-based sintered magnets can be produced by the following production process, for example.
(1) In a pulverizing step, an R-T-B-based magnet material alloy is hydrogen decrepitated (coarsely pulverized) and then finely pulverized in a jet mill or the like. In this manner, a fine powder is obtained.
(2) In a forming step, the obtained fine powder is pressed in a magnetic field to be formed into a green body.
(3) In a sintering step, the pressed green body is sintered in a vacuum and then the sintered body is subjected to a heat treatment (tempering). In this manner, an R-T-B-based sintered magnet is produced.
The demand for neodymium sintered magnets has been increasing worldwide in view of environmental protection (realization of low-carbon society), energy conservation, and use in next generation automobiles, high performance electronic devices, and the like. However, one problem with neodymium sintered magnets is their low coercive force at elevated temperatures.
To solve this problem, a type of neodymium sintered magnet, made from a magnet material alloy to which heavy rare earth elements (e.g., Dy, Tb, etc.) have been added as a partial replacement for Nd, has been developed and put into practical use. The amount of heavy rare earth elements added thereto is, for example, about 1 to 5 atomic % in total.
However, heavy rare earth elements pose a problem with regard to steady supply because of the limited deposits and uneven distribution of the resources. Thus, there is a need for a magnet material alloy capable of ensuring excellent coercive force in neodymium sintered magnets even in the case where the amount of heavy rare earth elements added to the magnet material alloy is reduced, specifically, in the case where the amount of heavy rare earth elements added is about 0 to 3 atomic % in total, for example.
In the past, various proposals have been made on R-T-B-based magnet material alloys as disclosed, for example, in Patent Literature 1. In the magnet material alloy proposed in Patent Literature 1, the volume percentage of the region containing an R2T17 phase having an average grain diameter of 3 μm or less in the short axis direction is from 0.5 to 10%. It is stated therein that, by using the magnet material alloy as a material for sintered magnets, it is possible to provide the resultant sintered magnets with a stably increased coercive force and therefore excellent magnetic properties.